White porous polyester films have been used in printing, labeling, electronics and display applications, because of their excellent whiteness, reflectance, and masking properties. Such a polyester film has been generally prepared by incorporating an organic or inorganic additive for forming voids. For example, JP Patent Laid-Open No. S58-50625 suggests the use of a foaming agent for generating voids, and JP Application Laid-Open No. S57-49648, the use of a polyolefin resin.
However, the method of using the foaming agent is hard to generate uniform-sized voids when the processing temperature is not carefully controlled. In addition, the polyolefin resin is not miscible with a polyester resin during extrude molding and drawing processes, and thus, uniform pore-formation cannot be achieved, which leads to film breakage during the drawing process for preparing the film, leading to unsatisfactory whiteness, reflectance and masking properties.
In order to overcome such problems, there has been reported a technique to add a compatibilizer to a polyester resin, but this technique suffers from such problems as low heat-stability of the compatibilizer and undesired migration thereof to the film surface, which limits its industrial applications.
Further, films having various functions have been used as the reflection film of a liquid crystal display (LCD), but bending and unwanted dark lines in the back light unit (BLU) appear due to specular reflection effects (not scattering reflection effects) of inorganic materials incorporated therein.